It is desirable to improve the heat transfer rates in heat exchangers such as air conditioners, furnaces, and in other apparatus which requires the efficient exchange of heat between a fluid and the wall over which the fluid flows. The effectiveness of the geometry of the convective heat transfer surfaces of such apparatus in producing efficient heat exchange with a minimal amount of friction losses can influence the required size and thus the initial cost of such apparatus, as well as operating costs and pumping power requirements. In applications where the heat exchanging geometry is for the purpose of reducing the temperature of the structure to permit it to operate in a hot environment, such as internal cooling geometries for gas turbine engine turbine airfoils, more efficient heat exchangers can reduce the needed mass flow rate of coolant, allow the apparatus to operate in a hotter environment, or permit the use of less exotic, less costly materials.
It is known that a fundamental contributor to the limiting of local convective heat transfer is the rapid growth and persistence of thermal boundary layers within internal flow passages of heat exchangers. The boundary layer acts as a thermal insulator between the wall and the flowing fluid. For this reason numerous geometrical schemes have been devised to disrupt this boundary layer and its insulating effect. Among these schemes have been the introduction of tabs, slits, and other flow disturbing elements and geometries to generate random and ordered velocitY fluctuations which increase heat transfer coefficients locally; however, excess pressure drops are created across these devices. When large numbers of these flow disturbing elements are used, which is often the case, a significant increase in the total pressure drop through the apparatus is incurred which requires increased fluid pumping power needs that offset some of the benefits of improved heat transfer. Additionally, such flow disturbing elements may be difficult and costly to fabricate.